linux-kernel - Re: [PATCH] mm/swapops: make is_pmd_migration

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Message-ID: <YmrfynZf7bU0Uoys@xz-m1.local>
Date:   Thu, 28 Apr 2022 14:41:14 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Hongchen Zhang <zhanghongchen@...ngson.cn>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swapops: make is_pmd_migration_entry more strict

On Thu, Apr 28, 2022 at 03:35:33PM +0800, Hongchen Zhang wrote:
> a pmd migration entry should first be a swap pmd,so
> use is_swap_pmd(pmd) instead of !pmd_present(pmd).
> 
> On the other hand, some architecture (MIPS for example)
> may misjudge a pmd_none entry as a pmd migration entry.
> 
> Signed-off-by: Hongchen Zhang <zhanghongchen@...ngson.cn>

The change looks reasonable,

Acked-by: Peter Xu <peterx@...hat.com>

Two more pure questions..

  (1) is there a real issue to be fixed with it?  Asked because I see most
      calls to is_pmd_migration_entry() should already make sure it's not
      none, and,

  (2) why it matters with MIPS?  Firstly this function is not used in arch
      specific code, and iiuc thp migration is not enabled at all for mips.

Thanks,

> ---
>  include/linux/swapops.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index d356ab4..1d16569 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -304,7 +304,7 @@ static inline pmd_t swp_entry_to_pmd(swp_entry_t entry)
>  
>  static inline int is_pmd_migration_entry(pmd_t pmd)
>  {
> -	return !pmd_present(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
> +	return is_swap_pmd(pmd) && is_migration_entry(pmd_to_swp_entry(pmd));
>  }
>  #else
>  static inline void set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
> -- 
> 1.8.3.1
> 

-- 
Peter Xu